To install place the folder in the steam folder
that is you'r steam username.
This is normaly found at
For XP users:
C:\Program Files\Steam\Steamapps\(you'r steam name here)
For win7 users:
c:\Program Files (x86)\Steam\Steamapps\(you'r steam name here)
and For Mac users:
Wait... you play games on a Mac? LOL!